This invention relates to a method of rapid and highly accurate analysis of oxygen and oxides that are contained in a metallic material by an optical emission spectroscopic analysis. More particularly, this invention relates to a quick and efficient analytical technology for quantitative determination of oxygen and/or the mean composition of oxides contained in a metallic material such as a steel material.
Oxides and so-called xe2x80x9coxide-type inclusionsxe2x80x9d, if contained in a steel material, greatly affect quality characteristics of the steel material such as the formation process characteristics of the steel material and its surface property. Therefore, when the steel material is molten, these oxides are controlled as the oxygen concentration of the molten steel is analyzed and is regulated to be in a predetermined concentration range.
A combustion-infrared absorption method has been generally employed at present as a quantitative determination method of this oxygen concentration. In other words, molten steel collected from a converter or a secondary refining furnace such as a VOD or an RH is solidified first in a mold to obtain an analytical specimen having a predetermined size. The specimen so obtained is loaded into a carbon crucible, and is heat-molten in an inert gas atmosphere. Oxygen dissociates from the oxides inside the specimen during this melting process, reacts with carbon of the crucible and generates a carbon dioxide gas. The oxygen concentration in the steel can be quantitatively determined as the quantity of carbon dioxide is measured by infrared absorption.
On the other hand, an optical emission spectroscopic analysis capable of analyzing simultaneously and quickly multiple elements has been employed for a process control analysis of a steel making process in order to analyze element concentrations of the metal specimen. This optical emission spectroscopic analysis executes a discharge operation a plurality of times between the metal specimen and an opposing electrode in an inert gas atmosphere, analyzes spectrally the spectrum inherent to each element generated by the discharge, and quantitatively determines each element concentration in the metal specimen from the intensity of each spectrum.
In the quantitative determination of the oxygen concentration in the steel by this optical emission spectroscopic analysis, however, emission spectra from trace amounts of moisture and oxygen of air that are contained in the discharge gas form a large background to the emission spectrum of oxygen as the object of the analysis from the metal specimen. In consequence, a signal-to-noise ratio (S/N) is low and accuracy of quantitative determination of oxygen is not sufficient, so that the optical emission spectroscopic analysis cannot be put into practical application. In the process control analysis of the steel making process described above, therefore, the analysis of only oxygen is conducted by the combustion-infrared absorption method described above. Therefore, excessive equipment and analysis time are necessary, and they present a problem in both economy and process management.
Quantitative determination by optical emission spectroscopic analysis can keep the accuracy of the determination of those inclusion elements which are substantially solid and insoluble such as Ca and Mg to a predetermined level. When the elements have high solid solubility such as Al, Si, Mn, Ni, Cr, and the like, however, it becomes difficult to distinguish the resulting spectral intensity originating from the solid solution elements from the spectral intensity from the solid insoluble elements. In consequence, accuracy of quantitative determination greatly relies on the skill or experience of analyzing engineers, and the problem of steadiness remains unsolved.
Further, the optical microscope observation method is one of the methods of evaluating the inclusions in the steel. This method observes a mirror-polished specimen through an optical microscope and counts the number of the inclusions contained in the specimen with eye. However, this method requires one- or two-days of time for the preparation of the specimen and for the measurement, lacks quickness, and cannot determine quantitatively the composition of the inclusions because the method is an inspection method of cleanness with eye.
Another evaluation method identifies the inclusions in the steel with an electron probe micro-analyzer (EPMA) or an electron microscope. However, this method not only requires a long time for polishing as a pre-treatment of the specimen but complicated procedures for the operation of equipment and various processing. Though this method can quantitatively determine the composition, it lacks quickness of measurement and cannot analyze quickly large quantities of specimens.
An extraction separationxe2x80x94ICP (inductively coupled plasma) atomic emission spectroscopic analysis has been employed in recent years as a composition analysis method of the inclusions in the steel. This method chemically extracts the oxides by dissolving the specimen in an acid or halogen solution, further dissolves the oxides obtained as the extraction residue and analyzes the composition by the ICP optical emission spectroscopic analysis. Because one- or two-days of time is necessary for the pre-treatment and the measurement of the specimen, this method lacks quickness of measurement and involves further the problem that the result of the analysis varies depending on the kind of the extracting solution selected.
As described above, a plurality of analyzing means have been used in combination with one another in the past depending on the object of analysis in the analysis of the oxygen concentration and/or the oxide composition of inclusions in the metallic specimen in order to keep a predetermined level of accuracy of quantitative determination. Because the analyzing time also varies from specimen to specimen, the result of analysis of all the data cannot be obtained before a long time of at least two days and the overall judgement is likely to get delayed. Consequently, all these methods are not free from the losses in both economy and time.
In view of the problems described above, the present invention provides a quantitative determination method capable of determining quickly and highly accurately a mean oxygen concentration in a metallic material and/or a mean inclusion composition by using, as analyzing means, only an optical emission spectrometer alone as a main analytical apparatus for the process control analysis of a steel making process.
To accomplish this object, the inventors of the present invention have paid specific attention to the form in which the elements are present in a metallic specimen and their spectral intensities obtained by emission spectroscopy, have examined their correlation, and have thus completed the present invention.
In other words, the present invention provides a method of quantitatively determining the mean oxygen concentration in a metallic material and/or the mean oxide-forming element concentration originating from oxides in the metallic material, which method comprises the steps of:
(1) conducting a discharge operation a plurality of times between the metallic material and an opposing electrode in an inert gas atmosphere to obtain an optical emission spectroscopic spectrum;
(2) selecting a discharge from the spectrum in which at least the oxide-forming element spectrum exceeds a predetermined intensity;
(3) subtracting a background from the intensity of the oxygen spectrum and/or the intensity of the oxide-forming element spectrum in the selected discharge, and accumulating the balances to obtain a cumulative spectrum intensity; and
(4) executing quantitative determination by a calibration curve method.
The present invention provides also a method of quantitatively determining a mean oxide-forming element concentration originating from oxides in the metallic material described above, which method comprises the steps of:
(1) selecting a discharge in which an oxide-forming element spectrum and the oxygen spectrum exceed respective predetermined intensities;
(2) subtracting the predetermined intensity of the oxide-forming element as a background from the intensity of the oxide-forming element spectrum in the selected discharge, and accumulating the balances to obtain a cumulative spectral intensity of the oxide-forming elements; and
(3) conducting quantitative determination using a separately prepared calibration curve of the average concentration of oxide-forming elements, comparing this to the above cumulative spectral intensity.
The present invention provides further a method of quantitatively determining a mean oxygen concentration in the metallic material described above, which method comprises the steps of:
(1) selecting a discharge in which the oxide-forming element spectrum exceeds a predetermined intensity;
(2) selecting oxygen spectra obtained at the time of the discharge so selected;
(3) determining a background of the oxygen spectrum, through the frequency distribution of the other oxygen spectra, using the intensity and the number of discharges;
(4) selecting again from among the oxygen spectra selected by the steps (2) the oxygen spectra having an intensity higher than the background;,
(5) subtracting the background from the intensity of the oxygen spectrum selected again and accumulating the balances to obtain a cumulative spectral intensity of oxygen; and
(6) conducting quantitative determination using a calibration curve, prepared separately from the oxygen concentration and the cumulative spectral intensity.
Incidentally, in either of the methods of quantitatively determining the mean oxygen concentration in the metallic specimen and/or the mean oxide-forming element concentration of oxides in the metallic material according to the present invention described above, the quantitative determination method preferably finds the intensity ratio of the spectral intensity of the element to be quantitatively determined to the spectral intensity of the matrix element of the metallic material.
In either of the cases whether the mean oxygen concentration in the metallic material and/or the mean oxide-forming element concentration originating from oxides in the metallic material is quantitatively determined, the predetermined reference intensity preferably employs at least one of the following; median value, mean value, or mean value+nxc3x97standard deviation (n: positive integer) in a frequency distribution of the spectral intensity of the element to be determined and the number of discharges, or in a frequency distribution of the spectral intensity ratio of the element to be determined to the matrix element.
The method of the present invention which quantitatively determines the mean oxygen concentration in the metallic material and/or the mean oxide-forming element concentration of oxides in the metallic material can be employed appropriately when the metallic material is iron.